When using a laser beam to perform ocular surgery, the precise movement of the laser beam's focal point through the tissue to be altered is absolutely imperative. Specifically, focal point position accuracies within about ten microns (10 μm) are preferable. To do this, the desired path for the laser beam's focal point must have a precisely defined start point. And, the laser beam's focal point must then be moved along the prescribed path. Although this can be accomplished in some situations with open loop control (i.e. having the laser beam focal point follow a pre-programmed path), in many other situations it may be more desirable to incorporate a closed loop feedback control system. Unlike open loop systems, closed loop feedback control systems provide continuous monitoring and corrections for deviations of the focal point. In either case, movements of the laser beam's focal point must be accomplished in the context of a reference datum.
An important requirement for any closed loop feedback control system is the need to accurately identify an appropriate error signal. As implied above, this error signal must be measurable. Thus, a reference datum is required from which the error signal can be measured. Once the error signal is identified, control of the system's performance is made by system adjustments that will nullify, or at least minimize, the error signal. Stated differently, deviations (i.e. error signals) from desired performance parameters must be determinable and maintained below an acceptable minimum. For the specific case involving feedback control of a surgical laser's focal point during ocular surgery, a reference datum that is anatomically related to the eye undergoing surgery needs to be selected. Further, knowledge of the location of the laser beam's focal point relative to the reference datum, and thus relative to a path through the eye, is also required.
Anatomically, the eye includes various tissues that may be beneficially altered by laser surgery. These include: the cornea, the crystalline lens, and the retina. Importantly, a thorough knowledge of the geometry of these ocular elements, and of their geometrical relationship to each other, is essential for successful surgery. All of this, of course, cannot be done by merely an external examination of the eye. With this limitation in mind, one method for imaging the interior of an eye involves optical coherence tomography (OCT) techniques. Fortunately, these techniques are well known to skilled artisans (e.g. See U.S. Pat. No. 6,004,314 which issued to Wei et al. for an invention entitled “Optical Coherence Tomography Assisted Surgical Apparatus”). Specifically, for purposes of the present invention, OCT can be employed to identify an appropriate eye-based reference datum for conduct of the laser surgery. Further, OCT provides a means for visualizing a treatment area inside the eye, while laser surgery is being performed. Although OCT techniques may be preferred, it will be appreciated by the skilled artisan that other imaging techniques might be used for the purposes of the present invention. Specifically, imaging techniques such as confocal microscopy, Scheimpflug principle, or second harmonic generation microscopy, may be employed.
In light of the above, it is an object of the present invention to provide a method and apparatus for directing a surgical laser beam onto tissue in a treatment area of an eye of a patient, wherein control of the laser beam is based on cross-sectional views of the eye obtained by employing OCT techniques. Another object of the present invention is to provide a method and apparatus for directing a surgical laser beam onto tissue in a treatment area of an eye of a patient wherein an eye-based reference datum can be selected that is most appropriate for the particular surgical operation that is to be performed. Still another object of the present invention to provide a method and apparatus for directing a surgical laser beam onto tissue in a treatment area of an eye of a patient that is easy to implement, is relatively simple to manufacture, and is comparatively cost effective.